Method for the quantification of 227AC in 223RA compositions

ABSTRACT

A method for the quantification of  227 Ac in a  223 Ra composition comprising passing the composition through a first solid phase extraction column A, wherein said column comprises a thorium specific resin, passing the eluate of column A through a second solid phase extraction column B, wherein said column comprises an actinium specific resin and recovering the  227 Ac absorbed onto the resin in column B and determining the amount thereof.

FIELD OF THE INVENTION

The present invention relates to a novel method for quantifying levelsof ²²⁷Ac in ²²³Ra compositions, in particular a method which involvessolid phase extraction followed by quantification via the in-growth ofthe ²²⁷Th daughter via γ-spectrometry. The invention further relates tothe use of the method of the invention in determining the level of ²²⁷Acin a ²²³Ra composition and to an apparatus for use in the method of theinvention.

BACKGROUND

A substantial percentage of cancer patients is effected by skeletalmetastases. As many as 85% of patients with advanced lung, prostate andbreast carcinoma develop bony metastates (Garret 1993, Nielsen et al,1991). They are associated with a decline in health and quality of life,ultimately leading to death, often within a few years.

When tumors or metastases cannot be removed by surgery, the conventionalapproach is to apply external beam radiotherapy and chemotherapy. Bothsuffer from a lack of selectivity for tumor cells and tumor tissue. As aconsequence, treatment most often cannot be applied at curative levelsdue to toxicity to healthy tissue.

Bone-seeking β-emitters like ⁸⁹Sr and ¹⁵³Sm complexed withethylene-diaminetetramethylene-phosphonate (EDTMP) have been used asinternal radiotherapy agents in the pain palliation of painful bonemetastases especially in prostate cancer. The altered skeletal metabolicactivity around many bone metastases results in a local increase in boneformation and uptake of calcium, which is used to construct thehydroxyapatite bone mineral. Bone-seeking radionuclides target this boneadjacent to the tumor deposits. Calcium mimetics, such as strontium⁸⁹Sr, belong to the alkaline earth group of elements in the periodictable. They can be administered as an intravenous radioactive salt thatwill be incorporated into the newly formed hydroxyapatite in bonemetastases. Other radionuclides, such as ¹⁵³Sm, require a carriermolecule to achieve selective uptake to the bone, for example, EDTMP. Byselectively targeting areas of high metabolic activity in bone, a hightherapeutic index is possible.

However, the β-particles are characterized by low-linear energy transfer(LET) typically in the range of 0.2-1.0 keV/μm and a modest relativebiological effectiveness (RBE). The use of highly energetic β-particlesis restricted by the radiation burden and cell damage to surroundinghealthy tissue and especially by the suppression of blood cells in thered bone marrow. Hence, there is an unmet need for more effectivebone-targeted treatments that improve quality of life and survivalwhilst maintaining a favorable safety profile.

The use of α-emitting radionuclides has a major advantage inradiotherapy of cancer. Compared to the low LET values of β-emitters,α-emitters have a mean LET value of 80-100 keV/μm. ²²³Ra has shownparticular promise. For example, Alpharadin® (²²³RaCl₂) has completed aglobal phase-III clinical trial in patients with castration-resistantprostate cancer (CRPC) and bone metastases. Data shows that Alpharadinprolongs patient overall survival time while offering a well toleratedsafety profile (Brady et al, Cancer J., 2013, 19, 71-78). ²²³Ra, like⁸⁹Sr, is a calcium mimic and also an alkaline earth element and can beadministered as an intravenous radioactive salt. Due to the highLET-values of α-particles and, consequently, their short path-length inhuman tissue (<100 μm), a highly cytotoxic radiation-dose can bedelivered to targeted cancer cells, while damage to the surroundinghealthy tissue is limited.

Quality control is an essential part of pharmaceutical manufacture, toensure the drugs sent to the market are safe and therapeutically activeformulations have a performance which is consistent and predictable. Theterm quality control refers to the sum of all procedures undertaken oneach batch to ensure e.g. the identity, activity and purity.

Radionuclidic purity is defined as the percentage of a contaminatingradionuclide relative to the wanted radionuclide e.g. ²²⁷Ac relative to²²³Ra with respect to activity in Bq. The primary reason for seekingradionuclidic purity in a radiopharmaceutical is to avoid unwantedadministration of radiation to the patient. It is therefore extremelyimportant to strictly control the levels of radionuclidic impurities inradiopharmaceuticals. Radionuclidic impurities may originate fromseveral sources. For example, when a parent-daughter radionuclidegenerator system is used to produce the radionuclide of interest, theparent nuclides are defined as impurities in the product. Actions mustbe taken during production to ensure that the parent nuclides areseparated from the nuclide of interest and, before release of thefinished product for human use, it has to be confirmed that theradioactivity of the radionuclidic impurities are below the limitspecified for the product.

Production of ²²³Ra for pharmaceutical use is typically based on aradionuclide generator where the mother nuclide ²²⁷Ac (t_(1/2)=21.77years) is adsorbed on a column material. The daughter radionuclides are²²⁷Th (t_(1/2)=18.68 days) and ²²³Ra (t_(1/2)=11.43 days). ²²³Ra isseparated by column elution. ²²⁷Ac and its daughter nuclide ²²⁷Th mustbe strongly retained under conditions were ²²³Ra can be eluted. ²²⁷Acand ²²⁷Th do not have the same bone seeking properties as ²²³Ra and areregarded as impurities. Even very low amounts of these nuclides cannotbe accepted in the pharmaceutical product. The acceptance criterion forAlpharadin has been set to not more than 0.004% for ²²⁷Ac and not morethan 0.5% for ²²⁷Th relative to ²²³Ra with respect to activity in Bq.Similar criteria would be expected for other ²²³Ra products. Prior toformal release of the product to patients, each produced batch ofradiopharmaceutical (e.g. Alpharadin) must be tested to show that itmeets the acceptance criteria (adequately defined identity, strength,quality and purity). Due to the inherently short half-life of ²²³Ra, theradiopharmaceutical may be released before completion of all tests (e.g.sterility testing). This naturally has the disadvantage that patientscould be exposed to a formulation which does not meet all the qualitycontrol criteria.

A quantitative determination of ²²⁷Ac is difficult as ²²⁷Ac decaysalmost entirely by emission of a low-energy β-particle (E_(β,max)=0.0448MeV), which is virtually undetectable in the presence of all theenergetic α- and β-emitters of the ²²⁷Ac chain (see FIG. 1). ²²⁷Ac alsodecays by α-emission in 1.38% of its disintegrations. However, directα-spectrometric determination of ²²⁷Ac is complicated by interferencesfrom the α-emissions of its rapidly growing decay products. Freshlypurified ²²⁷Ac emits no analytically useful γ-radiation.

Consequently, many radiometric methods determine ²²⁷Ac indirectly bymeasurements of the α- and γ-radiations of its daughters, in particularby high-resolution γ-spectrometry of its daughter ²²⁷Th. However, thiscannot be determined until 10-12 months after release of the product asanalysis must wait until there are sufficiently measurable levels of²²⁷Th. At this time, the potential amount of ²²⁷Ac contamination is inequilibrium with its daughter ²²⁷Th. Furthermore, the initial amounts of²²³Ra and any ²²⁷Th in the product would have decayed completely. Thesedisadvantages not only lead to inaccuracy of results and increased costsbut, more significantly, mean that the result comes too late for the²²³Ra pharmaceutical to be withdrawn from release to patients should itbe shown to be contaminated with ²²⁷Ac at levels which would beconsidered to jeopardise the efficacy of the treatment or the safety ofthe patient.

In view of the above, there remains a need to develop a new, reliable,accurate and cost-effective radiochemical method for early determinationof the potential contamination of ²²⁷Ac in ²²³Ra pharmaceuticals, suchas Alpharadin (RaCl₂). In particular, it would be an advantage toproduce a method which is able to give a result in a matter of daysrather than months. Ultimately, an analysis method which can becompleted prior to release of the product and its administration topatients is attractive. The following criteria set out the desirablefeatures of a new quantification method:

-   -   1. ²²⁷Ac should selectively be separated from the precursors.    -   2. Recovery of ²²⁷Ac>70% and precision>30%    -   3. Robustness i.e. the analytical result should remain        unaffected by small variations in method parameters.    -   4. Easy to operate in routine production (in terms of time and        cost).    -   5. Sample activity should be as low as possible due to cost and        radiation exposure to the operators, and/or    -   6. Separation and quantification should be fulfilled before        release of the product i.e. within 2 days after production of        the ²²³Ra pharmaceutical (e.g. 223-radium chloride).

The present inventors have surprisingly found that an analytical methodemploying a tandem column arrangement comprising two different solidphase extraction resins can fulfil some or all of these requirements. Inparticular, the two columns enable facile separation and isolation of²²⁷Ac, which can be rapidly quantified.

SUMMARY OF THE INVENTION

Thus, viewed from one aspect, the invention provides a method for thequantification of ²²⁷Ac in a ²²³Ra composition, said method comprising:

(i) passing said ²²³Ra composition through a first solid phaseextraction column A, wherein said column comprises a thorium specificresin (e.g. dipentyl pentylphosphonate UTEVA resin);

(ii) passing the eluate of column A through a second solid phaseextraction column B, wherein said column comprises an actinium specificresin (e.g. N, N, N′, N′-tetra-n-octyldiglycolamide DGA resin);

(iii) recovering the ²²⁷Ac absorbed onto the resin in column B anddetermining the amount thereof.

Viewed from another aspect the invention provides a method ashereinbefore described, said method comprising

-   (i) Placing a first solid phase extraction column A comprising a    thorium specific resin (e.g. dipentyl pentylphosphonate UTEVA resin)    and a second solid phase extraction column B comprising an actinium    specific resin (e.g. N, N, N′, N′-tetra-n-octyldiglycolamide DGA    resin) in series, preferably wherein the output of column A is    connected to the input of column B;-   (ii) Adding a volume of a ²²³Ra composition corresponding to a known    activity (e.g. 15 MBq) of ²²³Ra to an equal volume of nitric acid,    preferably 8 mol/L nitric acid;-   (iii) Transferring the sample from step (ii) to the input of the    column A;-   (iv) Passing said sample through both columns A and B-   (v) Washing both columns with 20-100 times the combined volume of    the two columns (e.g. 5-10 ml) nitric acid, preferably 4 mol/L    nitric acid;-   (vi) Disconnecting column A from column B;-   (vii) Washing column B with 40-200 times its volume (e.g. 5-10 ml)    nitric acid, preferably 4 mol/L nitric acid;-   (viii) Washing column B with 40-200 times its volume (e.g. 5-10 ml)    nitric acid at a concentration less than that used in step (vii),    such as 0.05 mol/L nitric acid.-   (ix) Determining the amount of ²²⁷Ac present in the eluate from    column B obtained in step (viii).

Viewed from another aspect the invention provides the use of a method ashereinbefore described in the quantification of ²²⁷Ac in a ²²³Racomposition.

Viewed from another aspect the invention provides apparatus for use in amethod as hereinbefore described, wherein said apparatus comprises afirst solid phase extraction column A, wherein said column comprises athorium specific resin (e.g. a dipentyl pentylphosphonate UTEVA resin),and a second solid phase extraction column B, wherein said columncomprises an actinium specific resin (e.g. a N, N , N′,N′-tetra-n-octyldiglycolamide DGA resin).

DEFINITIONS

The ²²³Ra composition of the invention will be understood to be anycomposition which comprises the radionuclide ²²³Ra. The composition willtypically be a pharmaceutical composition or a precursor to apharmaceutical solution and will therefore usually contain theadditional components often found in such compositions, e.g.pharmaceutically acceptable diluents, excipients and carriers. Suchcomponents are well known in the art. The ²²³Ra may be in any form,however the most preferred form is as a salt such as a halide salt,preferably RaCl₂ (Alpharadin®), optionally in combination with other Rasalts. It will be appreciated that in order to be compatible with themethod of the invention the 223Ra composition must be in solution,typically an aqueous solution, such as an aqueous acid solution.

The method of the invention employs solid phase extraction. Thistechnique is well known in the art, however a brief outline is providedhere for completeness.

Solid-Phase Extraction (SPE) has become widely accepted as a substitutefor traditional liquid-liquid extraction (LLE) in many types ofseparation procedures, and especially for those involving low toultralow concentrations of analyte. SPE is based on the same principlesas solvent extraction, which often involves complexation to form alipophilic compound of the analyte followed by transfer of this compoundinto an organic phase. In SPE the non-aqueous phase is solid instead ofliquid as it is in LLE. SPE is generally faster, more efficient andgenerates less waste than LLE.

SPE comprises three major components; an inert support, a stationaryphase and a mobile phase. The inert support usually consists of poroussilica or particles of an organic polymer ranging in size from 50 to 150μm in diameter. The stationary phase, which is on the surface of theinert support, is selected appropriately depending on the analytesinvolved. The mobile phase is usually an aqueous acid solution, e.g.nitric or hydrochloric acid.

The method of the invention employs two different stationary phases(resins).

The first resin is a thorium specific resin, typically an UTEVA Resin(Uranium and TEtraValents Actinides), which is mainly used for theseparation of uranium and tetravalent actinides. The extractant coatedon the inert support is selected to specifically bind thorium in asolution mixture of radium, thorium and actinium. This specificity maybe under all conditions, or the conditions used in the methods of theinvention may be chosen to ensure specificity.

Extractants suitable for thorium specific resins include phosphonates,particularly alkyl phosphonates. Dialkyl alkyl phosphonates such asthose of the following formula (Formula I) are preferred:

wherein each of R₁-R₃ is independently a C₃-C₈ straight or branchedchain alkyl group. Preferably R₁-R₃ are straight chain alkyl groups.Preferably R₁ is a C₄-C₆ straight chain alkyl group, most preferablyn-pentyl. R₂ and R₃ may be identical or different. Preferably R₂ and R₃are identical. Preferably each of R₂ and R₃ is a straight chain C₄-C₆alkyl group, most preferably n-pentyl. A high preferably extractant isdipentyl pentylphosphonate, which has the following structure:

The second resin is an actinium specific resin, typically selected tospecifically bind actinium in a solution mixture of radium and actinium.This specificity may be under all conditions, or conditions used in themethods of the invention may be selected to ensure specificity.

In some embodiments, the conditions may be such that the actiniumspecific resin has some degree of affinity for radium as well asactinium and under those conditions both radium and actinium may bind tothe second resin. It will be appreciated that, under such circumstances,the method of the invention may require a further step in which theconditions are altered such that any radium which has bound to thesecond resin may be specifically eluted whilst the actinium remainsbound to the resin, before the actinium may be eluted from the secondresin.

Preferably, the conditions used in the methods of the invention arechosen such that the second resin does not have any affinity for radiumand only actinium binds to the second resin. Thus, in a preferableembodiment, the conditions used in the method of the invention are suchthat the second resin is specific for actinium. A resin may beconsidered “specific” for one element over another if that resin willretain at least 90% of the first element under conditions that wouldelute at least 90% of the second element. This is preferably 95%, morepreferably 99%. Typically, the conditions chosen in the methods of theinvention are certain concentrations of mineral acids (e.g. nitric acid)in water.

Extractants suitable for actinium specific resins includediglycolamides, particularly tetra-alkyl diglycolamides of the followingformula (Formula II):

wherein R₁-R₄ are independently C₃-C₁₂ straight or branched chain alkylgroups, preferably C₅-C₁₀ straight or branched chain alkyl groups. R₁-R₄may be identical or different, preferably identical. R₁-R₄ may all be C₈alkyl groups. A preferred example isN,N,N′,N′-tetra-n-octyldiglycolamide (DGA Resin, Normal), which has thefollowing structure:

wherein the R-groups are straight chain C₈ alkyl groups. Thecorresponding resin where the R-groups are branched C₈ alkyl groups isalso of value.

In the context of the invention, the term “eluate” refers to thesolution of solvent and dissolved matter resulting from elution, i.e.the mixture of components which elutes following separation using asolid phase extraction column.

The term “eluent” should be understood to be interchangeable with theterm “mobile phase”. Both terms are well known in the art and are usedto refer to the solvent which is passed through a solid phase extractioncolumn and is used to effect separation.

DETAILED DESCRIPTION

The method of the invention comprises the following steps

(i) passing a ²²³Ra composition (e.g. one containing ²²⁷Ac and ²²⁷Thcontaminants) through a first solid phase extraction column A, whereinsaid column comprises a thorium specific resin (e.g. a dipentylpentylphosphonate UTEVA resin);

(ii) passing the eluate of column A through a second solid phaseextraction column B, wherein said column comprises an actinium specificresin (e.g. a N, N, N′, N′-tetra-n-octyldiglycolamide DGA resin);

(ii) recovering the ²²⁷Ac absorbed onto the resin in column B anddetermining the amount thereof.

In a preferable embodiment, column A and column B are arranged in seriessuch that the eluate from column A passes directly into column B, i.e.wherein the output of column A is connected to the input of column B.The most preferable arrangement is for column A to be positioned abovecolumn B such that the eluate from column A drains directly into columnB.

The method of the invention relies on the surprising finding that bychoice of a resin and column configuration, contaminant ²²⁷Ac can bepurified from a mixture of ²²³Ra and ²²⁷Th to a sufficient degree toallow for accurate measurement of the ²²⁷Ac via ²²⁷Th in-growth. Forexample, a UTEVA resin is capable of selectively retaining ²²⁷Th out ofa mixture of ²²³Ra, ²²⁷Ac and ²²⁷Th and moreover that a DGA resin iscapable of selectively retaining ²²⁷Ac from a mixture of ²²⁷Ac and²²³Ra. This results in an efficient separation method. An outline of theprocess is provided in FIG. 2.

The ²²³Ra composition used in the method of the invention comprises²²³Ra. It will typically also comprise both ²²⁷Th and ²²⁷Accontaminants. Thus, all three radionuclides are usually present in thestarting mixture of analytes. As the mixture passes through the firstcolumn A, any ²²⁷Th present will absorb onto the thorium specific resin(e.g. UTEVA resin), leaving only ²²³Ra and ²²⁷Ac present in the eluate.As this eluate passes though the second column B, any ²²⁷Ac will absorbonto the actinium specific resin (e.g. DGA resin). The ²²³Ra willtypically remain in the mobile phase. In some embodiments, the actiniumspecific resin may be washed with additional volumes of mobile phase soas to ensure all ²²³Ra is eluted. Thus the total ²²⁷Ac fraction may beobtained and isolated.

Importantly, the ²²⁷Ac fraction, which is bound to the actinium specificresin (e.g. DGA resin), will be substantially free, preferablycompletely free, of ²²⁷Th and ²²³Ra, thereby enabling more faciledetermination of its quantity via detection of the in-growth of itsdaughter nuclide, ²²⁷Th at very low levels. In particular, results willnot be skewed by levels of ²²⁷Th initially present in the ²²³Racomposition or masked by interferences due to other, more energetic,decay chains beginning at ²²⁷Th or ²²³Ra. A first isotope may beconsidered “substantially free” of a second isotope if the secondisotope is present at a concentration of less than 1%, preferably lessthan 0.01%, relative to the concentration of the first isotope.Correspondingly, “completely free” may be considered to correspond to aconcentration of less than 0.001% of the second isotope relative to thefirst isotope.

The mobile phase (eluent) is typically a solution comprising an acid,such as hydrochloric acid or nitric acid. The most preferable acid isnitric acid. Typically, the concentration of any acid used in the methodof the invention will be in the range 0.01 to 10 mol/L, preferably 0.02to 8 mol/L, such as 0.05 to 4 mol/L.

Column A comprises a thorium specific resin, such as a UTEVA resin. Theinventors have found that the affinity of an UTEVA resin for ²²⁷Thincreases with increasing nitric acid concentration. This is thought toarise because as the concentration of the nitric acid increases so toodoes the propensity with which the ²²⁷Th will form nitrate complexes. Itis believed to be these complexes for which the resin has affinity.Column B comprises an actinium specific resin, such as a DGA resin. DGAresin has been found to have particular affinity for ²²⁷Ac.

The ²²³Ra composition used in the methods of the invention typicallycomprises ²²³Ra at a concentration in the range 2 to 30 MBq/ml (e.g. 2.4to 30 MBq/ml), such as 5 to 20 MBq/ml. The composition will usually beused in the form of an aqueous acid solution, such as nitric acid. Theacid will typically have a concentration in the range 4-10 mol/L, forexample, 8 mol/L.

In step (iii) the ²²⁷Ac absorbed onto the resin of column B is removed.This may be carried out by a variety of methods but is typicallyachieved by washing the column with an aqueous acid solution of lowerconcentration than that which was used as eluent in step (ii), such as0.05 mol/L nitric acid. The volume of aqueous acid solution used to washthe column may be in the range 16 to 400 times the volume of the column(e.g. 2-20 ml), preferably 40-200 times (e.g. 5-10 ml). The eluateobtained from column B after step (iii) contains ²²⁷Ac. Preferably thiseluate is substantially free of ²²⁷Th. For example, the eluate maycontain ²²⁷Th at a molar concentration of less than 5%, preferably lessthan 1% or less than 0.1% and more preferably less than 0.01% relativeto the concentration of ²²⁷Ac.

In a highly preferred embodiment, the method of the invention comprisesthe following steps:

-   (i) Place a first solid phase extraction column A comprising a    thorium specific resin (e.g. a dipentyl pentylphosphonate UTEVA    resin) and a second solid phase extraction column B comprising an    actinium specific resin (e.g. a N, N, N′,    N′-tetra-n-octyldiglycolamide DGA resin) in series, preferably    wherein the output of column A is connected to the input of column    B;-   (ii) Add a volume of a ²²³Ra composition corresponding to a known    activity (e.g. 15 MBq) of ²²³Ra to an equal volume of nitric acid,    preferably 8 mol/L nitric acid;-   (iii) Transfer the sample from step (ii) to the input of the column    A;-   (iv) Pass said sample through both columns A and B-   (v) Wash both columns with 20-100 times the combined volume of the    two columns (e.g. 5-10 ml) nitric acid, preferably 4 mol/L nitric    acid;-   (vi) Disconnect column A from column B;-   (vii) Wash column B with 40-200 times its volume (e.g. 5-10 ml)    nitric acid, preferably 4 mol/L nitric acid;-   (viii) Wash column B with 40-200 times its volume (e.g. 5-10 ml)    nitric acid at a concentration less than that used in step (vii),    such as 0.05 mol/L nitric acid.-   (ix) Determining the amount of ²²⁷Ac present in the eluate from    column B obtained in step (viii).

Following isolation of the ²²⁷Ac from the actinium specific resin (e.g.DGA resin), its amount may be quantified by any known method in the art.Typical percentage recoveries of ²²⁷Ac using the method of the inventionare in the range 70-100%, such as 72-98%, preferably 74-97% (e.g. 80 to97% or 80 to 90%). Evidently, for an analytical method reproducibilityin recovery of ²²⁷Ac is as important as the absolute recovery. Thedistribution of such recoveries will thus typically have a standarddeviation of no more than 20%, preferably no more than 10%.

Typical methods used to determine the quantity of ²²⁷Ac may involveγ-spectrometry, α-spectrometry and liquid scintillation counting (LSC)with pulse-shape discrimination. The preferred technique isγ-spectrometry, which enables quantification of ²²⁷Ac via in-growth anddetection of the daughter ²²⁷Th. Methods for performing γ-spectrometryare well known in the art.

The activity of ²²⁷Ac, which is not directly determinable by γ-rayspectrometry, can be calculated from measurement of the daughter ²²⁷Th.As the specification limit for a ²²³Ra pharmaceutical is 0.004% ²²⁷Ac,relative to ²²³Ra, an activity of 15 MBq ²²³Ra should give an activityof 600 Bq of ²²⁷Ac. The in-growth of ²²⁷Th from ²²⁷Ac is calculated byEquation 1.A _(ingrowth)(²²⁷Th)=A ₀·(1−e ^(−λ) ^(Th-227) ^(t))   (1)

Due to regulatory requirements for radiopharmaceuticals, the result fromradionuclidic purity of a ²²³Ra pharmaceutical should be availablebefore release of the product. In order to meet these requirements themaximum in-growth period of ²²⁷Th from ²²⁷Ac should preferably not bemore than two days to avoid a prohibitively high loss of ²²³Ra by decaybefore administration. The calculated activity after 24 and 48 hoursin-growth of ²²⁷Th from 600 Bq ²²⁷Ac is shown in Table 1.

TABLE 1 Ingrowth of ²²⁷Th from 600 Bq ²²⁷Ac Hours after separationIngrowth ²²⁷Th (Bq) 24 21.9 48 42.9

After separation of ²²⁷Ac from ²²⁷Th and ²²³Ra, potential traces of²²⁷Th can be left in the sample (minimum detectable value<1.6 Bq). Bycounting only one spectrum, the activity of ²²⁷Ac can be overestimated.In the present invention, this issue has been addressed by utilizing twoconsecutive measurements of the ²²⁷Th daughter: one counted after 24hours and one after 48 hours from separation. ²²⁷Th activity obtainedfrom analyses after 24 hours is subtracted from ²²⁷Th activity obtainedafter 48 hours assuming that the in-growth of ²²⁷Th is almost linear inthe period. Correction of decay of potential traces of ²²⁷Th(t_(1/2)=18.68 days) between 24 and 48 hours after separation, has notbeen taken into account. It is considered to be sufficiently accurateand within the uncertainty of the measurement.

With the ²²⁷Th activity at measurement time one (24 h) and ²²⁷Thactivity at measurement time two (48 h), the unknown, but timeindependent, activity of the long-lived mother ²²⁷Ac can be calculatedby:A _(Δ)(²²⁷Th)=A _(spectrum2)(²²⁷Th)−A _(spectrum1)(²²⁷Th)   (2)

The activity of ²²⁷Ac in the sample at time 0, is based on in-growth of²²⁷Th. The equations used are given below.

$\begin{matrix}{{A_{0}\left( {\,^{227}{Ac}} \right)} = {{A_{\Delta}\left( {\,^{227}{Th}} \right)} \cdot \left( \frac{1}{1 - e^{{- \lambda_{{Th} \cdot 227}}t}} \right)}} & (3)\end{matrix}$where:

-   A₀(²²⁷Ac)=the activity of ²²⁷Ac in the ²²³Ra pharmaceutical (Bq)-   A_(Δ)(²²⁷Th)=the activity of ²²⁷Th produced between measurement time    one and measurement time two, e.g. 24 and 48 hours after separation    (Bq)-   λ_(Th-227)=ln 2/18.68 days

As seen from Table 1, the activity 24 and 48 hours after in-growth of²²⁷Th from 600 Bq ²²⁷Ac is 21.9 and 42.9 Bq, respectively. Based on thetheoretical calculation of the in-growth of ²²⁷Th and the fact that theseparation gives highly purified ²²⁷Ac it was assumed that a countingtime of 10000 s in the closest calibrated position (position 5 cm) fromthe detector surface was sufficient, and satisfactory countinguncertainties were achievable.

Consequently, in the methods of the invention, the quantity of ²²⁷Ac ispreferably determined by γ-spectrometry via the in-growth of thedaughter nuclide ²²⁷Th, wherein two measurements of the activity of the²²⁷Th daughter are made. Preferably these measurements are taken at nand 2n hours, wherein n is 12 to 36, preferably at 24 hours and 48hours, after performing the separation method of the invention. Activityis typically measured over a period of 10000 s at each time point.

In one variant, the method of the invention could be performed in thepresence of ²²⁵Ac as a tracer for the chemical yield of ²²⁷Ac to checkthe accuracy of the results. However, ²²⁵Ac is not commerciallyavailable hence it cannot be used in routine analysis at present.Instead, initial verification of the method may be carried out byspiking the ²²³Ra composition with ²²⁵Ac, providing quality control dataof critical process steps such as correct preparation of acid samples,weighting of correct resins and elution with the correct acid and acidconcentration. ²²⁵Ac can be presumed to have the same resin absorptionproperties as ²²⁷Ac but is easier to detect. ²²⁵Ac may be quantified bythe in-growth of the daughter ²¹³Bi via γ-spectrometry. The decay chainfor ²²⁵Ac is shown in FIG. 3.

The method of the invention is suitable for routine analysis of ²²⁷Ac in²²³Ra compositions and can be performed quickly on the same day aspreparation of the ²²³Ra composition. Preferably separation steps (i) to(iii) can be completed in no more than 2 hours, preferably no more than1 hour (e.g. 5 minutes to 1 hour). Advantageously, the results from themethod of the invention are typically available two days afterproduction i.e. before the product is released and administrated to thepatient.

The invention further relates to the use of the methods as hereinbeforedescribed in quantifying levels of ²²⁷Ac in ²²³Ra compositions. Itshould be appreciated that all previous discussion relating topreferable aspects of the invention relate equally to this embodiment.

The invention further relates to apparatus for use in a method ashereinbefore described. The apparatus comprises a first solid phaseextraction column A, wherein said column comprises a thorium specificresin (e.g. a dipentyl pentylphosphonate UTEVA resin), and a secondsolid phase extraction column B, wherein said column comprises anactinium specific resin (e.g. a N, N, N′, N′-tetra-n-octyldiglycolamideDGA resin). Preferably, column A and column B are arranged in series,preferably such that the output of column A is connection to the inputof column B. Most preferably, column A is positioned above column B suchthat the eluate from column A drains directly into column B. It shouldbe appreciated that all previous discussion relating to preferableaspects of the invention relate equally to this embodiment.

FIGURES

FIG. 1—Decay scheme of ²²⁷Ac to stable ²⁰⁷Pb. Branches with less than 2%probability are omitted.

FIG. 2—Process flow-chart for actinium, thorium and radium separationand purification using the method of the invention—extraction is shownusing aqueous HNO₃ of particular concentrations by way of example only.

FIG. 3—The decay scheme of ²²⁵Ac and daughter radionuclides to stable²⁰⁹Bi.

FIG. 4—HPGe γ-spectrum of ²²⁵Ac daughters ²²¹Fr and ²¹³Bi

FIG. 5—HPGe γ-spectrum of in-growth of ²²⁷Th from ²²⁷Ac 24 hours afterseparation from ²²³Ra-chloride drug substance

FIG. 6—Linearity of measured versus theoretical ²²⁷Th amount in ²²³Rachloride drug substance

EXAMPLES

The ²²³Ra composition utilised in the Examples is RaCl₂ (Alpharadin),hereinafter referred to as “Ra-chloride drug substance”.

Gamma-spectra were measured with a High-Purity Germanium detector (HPGe)of 50% efficiency (relative to a 3 inch×3 inch NaI detector for a ⁶⁰Cosource at a distance of 25 cm from the detector surface) coupled to a8192-channel Multi Channel Analyser (MCA). Spectra were analysed usingGammaVision Software (GammaVision-3.2 software, v 6.01, Ortec, OakRidge, USA). Calibration of the energy dependent efficiency of the HPGedetector at two fixed positions was performed with a reference source(γ-mixed standard from Eckert & Ziegler) with an overall uncertaintybelow 4%. The fixed calibration positions were 5 and 20 cm from thedetector. The HPGe detector was energy calibrated in an energy rangefrom 59-1400 keV.

In order to evaluate ²²³Ra with an activity in the MBq range, anionization chamber (“dose calibrator”, Capintec-CRC15-R) was used.Accurate activity measurements of radionuclides using commercialionization chambers require that the correct calibration setting (“dialsetting”) must be applied. For many nuclides, the manufactures of dosecalibrators, recommend those calibration settings.

²²³Ra is a relatively novel radionuclide in nuclear medicine and acalibration setting for the radionuclide is therefore not available fromcommercial manufactures of ionization chambers. A primarystandardization of ²²³Ra to establish dial settings was performed by theNational Institute of Standards and Technology (NIST). The reason is toassure quality-controlled measurements of the radioactivity of ²²³Raduring production, quality control and preparation of patient doses.

Measurements with ²²³Ra Standard Reference Material (SRM) from NIST wereperformed in Capintec dose calibrators. The determined calibrationsetting (dial setting) for the dose calibrator used in the presentinvention is presented in Table 2.

TABLE 2 ²²³Ra dose calibration setting ²²³Ra Dose Calibration SettingCapintec CRC-15R/serial no: (dial setting) Serial no 157623, B-lab,Algeta 262

The instruments were qualified, which means verification that theinstrument is installed correctly and is capable of operating asintended according to the specifications. Control of the HPGe instrumentwas performed daily before use by measuring the long-lived radionuclide²²⁶Ra. The radionuclides ⁵⁷Co and ¹³⁷Cs are used for daily control ofthe dose calibrator. The purpose of the quality control of theinstruments is to ensure that the instrument provides reliable andconsistent results.

Detection of ²²⁷Ac is difficult due to the low energy of its β-radiationand no useful γ-rays. Therefore, in order to easily obtain rapidinformation concerning the efficiency of the separation of ²²⁷Ac from²²⁷Th and ²²³Ra using the method of the invention the process wascarried out using ²²⁵Ac as a radioactive tracer in place of ²²⁷Ac. ²²⁵Accan be presumed to have the same resin absorption properties as ²²⁷Acbut is easier to detect. Examples 1-3 were carried out with ²²⁵Ac andExamples 4 and 5 with ²²⁷Ac. ²²⁵Ac may be quantified by the in-growth ofthe daughter ²¹³Bi via γ-spectrometry.

Uncertainties were calculated as follows:

Uncertainty in the Recovery (Examples 1, 2, 3 and 5)

There is an uncertainty σA in the activity of the “known” (spiked)samples, A.

There is an uncertainty σ_(B) in the activity of the “found” sample(eluate), B.

R = Recovery  (%) $R = {\frac{B}{A} \cdot 100}$$\sigma_{R} = {\sqrt{\frac{\sigma_{B}^{2}}{B^{2}} + \frac{\sigma_{A}^{2} \cdot B^{2}}{A^{4}}}.}$Uncertainty in the Deviation (Example 4)

There is an uncertainty σA in the calculated activity, A.

There is an uncertainty σB in measured activity, B.

$y = {\frac{A - B}{A} \cdot 100}$$\sigma_{y}^{\prime} = \sqrt{\left( \sigma_{A} \right)^{2} + \left( {\frac{A}{B^{2}}\sigma_{B}} \right)^{2}}$Calculation of the Combined Uncertainty (Example 5)

There is an uncertainty σ_(A) in the activity of the ingrowth of ²²⁷Thafter 24 hours, A.

There is an uncertainty σ_(B) in the activity of the ingrowth of ²²⁷Thafter 48 hours, B.

Calculation of the combined uncertainty:f=√{square root over (σ_(A) ²+σ_(B) ²)}

EXAMPLE 1 Separation of ²²⁵Ac from ²²⁷Th and ²²³Ra using Solid-PhaseExtraction Columns

Sample Preparation, ²²⁵Ac

The ²²⁵Ac used was supplied from the Institute for TransuraniumElements, Karlsruhe, Germany. The solution had a nominal total activityof 6 MBq ²²⁵Ac at the day of receipt and the activity was diluted in 4mol/L HNO₃ to an activity of 3.1 Bq/μL.

A known amount of ²²⁷Th and ²²⁵Ac was added to 15 MBq ²²³Ra-chloridedrug substance. The activity corresponded to approximately thespecification limits of ²²⁷Th and ²²⁷Ac in a ²²³Ra-chloride drugsubstance. The specification states that the activity of ²²⁷Th should beless than 0.5% of ²²³Ra activity and the activity of ²²⁷Ac should beless than 0.004% of ²²³Ra. Table 3 gives the activities of ²²⁵Ac and²²⁷Th used in the experiments.

TABLE 3 Activities (Bq) of ²²⁷Th and ²²⁵Ac in experiment I and II.Nuclide Experiment I Experiment II ²²⁷Th (kBq) 66.9 ± 2.2¹⁾ 74.6 ± 2.4¹⁾²²⁵Ac (Bq) 497.7 ± 23.9¹⁾ 667.8 ± 33.4¹⁾ ¹⁾Uncertainty in the activity(2 σ)Activities were determined using the following methods:

²²³Ra-chloride drug substance was transferred to two 20 mL vials (eachwith an activity of 15 MBq) and measured in a dose calibrator in dialsetting 262. The γ-rays of ²²⁵Ac and ²²⁷Th solutions were measured withan HPGe detector in the calibrated position 5 cm and 20 cm,respectively.

For determining the areas of the γ-peaks in the energy spectrum, ORTECGammaVision software was used. The energy and photon yield data aretaken from Evaluated Nuclear Data File (ENSDF—available fromhttp://www.nndc.bnl.gov/nudat2/—21 Jun. 2013). The most abundantγ-lines, given in Table 4, are used for the activity calculation.

TABLE 4 γ-ray energies and emission probabilities (percent) used for thedetermination of radionuclide activities. Library:Ra_223_DS_Ac_225_ENSDF_20_sep_2012.Lib Nuclide Energy Percent Half-LifeAc-225 99.80 1.0000 10 Days Th-229 124.55 0.6900 7932 Yrs. Th-229 136.991.1800 7932 Yrs. Ra-223 144.24 3.2700 11.43 Days Ra-223 154.21 5.700011.43 Days Th-229 156.00 1.1900 7932 Yrs. Th-229 204.70 0.6000 7932 Yrs.Th-229 210.85 2.8000 7932 Yrs. Fr-221 218.12 11.4000 4.9 Min. Th-227235.96 12.9000 18.68 Days Ra-223 269.46 13.9000 11.43 Days Ra-223 323.873.9900 11.43 Days Th-227 286.09 2.4000 18.68 Days Ra-223 338.28 2.840011.43 Days Bi-211 351.06 12.9200 11.43 Days Rn-219 401.81 6.6000 11.43Days Pb-211 427.09 1.7600 11.43 Days Bi-213 440.45 25.9400 45.59 Min.Ra-223 445.03 1.2900 11.43 Days Th-229 454.00 0.0100 7932 Yrs. Pb-211704.64 0.4620 11.43 Days Pb-211 832.01 3.5200 11.43 Days Data was takenfrom ENSDF.

Production of ²²⁵Ac is based on a ²²⁹Th generator from which ²²⁵Ac iseluted. The γ-lines from ²²⁹Th used for activity calculation are seen inTable 4. No traces of ²²⁹Th were found in the sample.

Preparation and Conditioning Procedure of the UTEVA and DGA Columns

The extraction-chromatographic resins as well as the prefilter materialwere packed in 2 ml disposable polystyrene plastic gravity-feed columns(obtained from Fisher Scientific). The following steps were carried out:

-   -   Weigh in approximately 100 mg of UTEVA resin and 50 mg of DGA        resin.    -   Transfer the resin to two 20 ml plastic vials and add        approximately 3 ml of 4 mol/L HNO₃ to each. Swirl to mix.    -   Prior to packing of the two columns, transfer a filter to the        columns. Push the filter down to the base of the columns.    -   Transfer the solution into the reservoir. Place a filter on the        top of the UTEVA resin and DGA resin.    -   Push the filter and the resin down.    -   Discard the acid above the top filter. Add 2-3 ml of 4 mol/L        HNO₃. Discard the acid again.    -   Remove the bottom plug from the columns.    -   Add 2-3 ml 4 mol/L HNO₃. Allow to drain.        Two-Column Separation Procedure    -   Place one UTEVA resin column and one DGA resin column in the        column rack in series, i.e., solutions from the UTEVA resin        column (on top) will drain into the DGA resin column (on bottom)        (see FIG. 1).    -   Based on ²²³Ra-chloride drug substance radioactivity        concentration, MBq/ml, pipette accurately a volume corresponding        to 15 MBq into a 20 ml vial. Add an equal amount of 8 mol/L        HNO₃.    -   Known activities of ²²⁷Th and ²²⁵Ac were added to the        ²²³Ra-chloride solution, according to Table 3.    -   Use a plastic pipette to transfer the sample to the top of the        UTEVA column. ²²⁵Ac and ²²³Ra will elute from the UTEVA column        into the DGA column while ²²⁷Th will absorb to the UTEVA column.    -   Wash the columns with 5 ml of 4 mol/L HNO₃.    -   Disconnect the UTEVA column from the DGA column.    -   Transfer 5 ml of 4 mol/L HNO₃ onto the top of the DGA column.        ²²³Ra will elute from the column while ²²⁷Ac will stay trapped        on the DGA column.    -   Elute the DGA column with 5 ml 0.05 mol/L HNO₃. This will remove        ²²⁷Ac from the column.

It will be appreciated that in routine analysis (separation of ²²⁷Ac,²²⁷Th and ²²³Ra) no addition of ²²⁷Th and ²²⁵Ac to the ²²³Ra-chloridesolution was carried out, the solution was transferred directly to theUTEVA column.

Two separation experiments with ²²⁵Ac were conducted. After completeseparation, the columns and eluates were counted the day afterseparation with an HPGe detector. ²²⁵Ac has no suitable γ-rays andtherefore it is quantified through its daughter ²¹³Bi. ²²⁵Ac is insecular equilibrium with is daughters after 24 hours. ²²⁵Ac wasidentified through the measurement of the daughters ²²¹Fr and ²¹³Bi. Aspectrum of the highly purified ²²⁵Ac eluate is shown in FIG. 4. Theγ-ray energies and intensities, used for the identification andquantification of ²²⁵Ac and ²²⁷Th, are presented in Table 4.

Percentage recovery for ²²⁵Ac obtained from experiments I and II was 97%and 86% respectively. Results are reported in Table 5. These resultsshow that the extraction resins UTEVA and DGA give an effective,reproducible, robust and rapid separation of ²²⁵Ac from ²²⁷Th and ²²³Ra.The DGA resin shows a strong retention of ²²⁵Ac in nitric acid andefficient stripping of ²²⁵Ac in dilute nitric acid. As seen from Table5, the breakthrough (“recovery” in Table 5) of ²²⁷Th and ²²³Ra in thefinal eluate was less than 2·10³ and 8·10⁴, respectively. Moreover, onlytraces of the initial amounts of ²²³Ra and ²²⁷Th were detected in theeluates. This shows that the separation procedure of ²²⁵Ac from ²²⁷Thand ²²³Ra with UTEVA and DGA column is highly effective. Thus, themethod demonstrates that separation of ²²⁷Ac, ²²⁷Th and ²²³Ra will alsobe effective.

TABLE 5 Activity (Bq) of ²²⁵Ac, ²²Th and ²²⁷Ra on the UTEVA and DGAcolumn after separation and in the eluate obtained by γ-rayspectrometry. Uncertainty in the activity (2 σ). Activities added at thestart of the experiment are seen in Table 3. Experiment I Experiment II²²⁵Ac 227_(Th) ²²³Ra ²²⁵Ac 227_(Th) ²²³Ra (Bq) (kBq) (Bq) (Bq) (kBq)(Bq) UTEVA N.D. 68.4 ± 2 N.D. <11.6¹ 68.4 ± 2 N.D. DGA 23.3 ± 3 N.D.20.5 ± 1 63.5 ± 1 N.D. <12.9¹ Eluate² 483.6 ± 20 <0.2¹ 41.5 ± 4 74.7 ± 6N.D. 84.4 ± 16 423.0 ± 20 <1.6¹ 21.6 ± 1  73.1 ± 5 <0.3¹ 8.4 ± 1Recovery (%)  97 ± 6 3 · 10⁻⁴ 2.8 · 10⁻⁴  86 ± 6 2.1 · 10⁻³ 7.6 · 10⁻⁴¹minimum detectable amount (MDA) ²In Experiment II, three fractions of1.7 ml of the eluate were collected and counted

EXAMPLE 2 Testing of the Robustness of the Method

Robustness of the method was tested by using new batches (new lotnumber) of UTEVA and DGA resin. The robustness of an analyticalprocedure is a measure of its capacity to remain unaffected by smallvariations in method parameters and provides an indication of itsreliability during normal usage.

A total of three experiments (I, II and III) were conducted with 15 MBq²²³Ra-chloride drug substance spiked with known activity of ²²⁵Ac. Theactivity of ²²⁵Ac is shown in Table 6. In addition, approximately 75 kBqof ²²⁷Th was added. The separation was performed according to theprocedure given in Example 1.

Percentage recovery of ²²⁵Ac obtained after eluting with 5 ml 0.05 mol/LHNO₃ was 68, 81 and 77%. The recoveries obtained were lower than for theUTEVA and DGA batches used in Example 1. It was therefore decided toincrease the eluting volume (in addition to the 5 ml already added) byadding 2 times a volume of 2.5 ml of 0.05 mol/L HNO₃. The results arepresented in Table 6. The percent recovery of ²²⁵Ac increased to 81, 85and 84% and this is comparable to the previously obtained result. Thisshows that the method is robust with regards to new batches (new lot) ofresins. However, the eluting volume may require adjustment to obtainsufficient recoveries (>70%).

TABLE 6 Measured ²²⁵Ac activity (Bq). Testing of robustness of themethod by using new batch of UTEVA and DGA resins. Experiment IExperiment II Experiment III ²²⁵Ac spike 652.4 ± 47 609.4 ± 52 712.6 ±50 Eluate (5 ml) 442.7 ± 35 491.2 ± 39 547.2 ± 43 Eluate (2.5 ml) 56.4 ±1 23.7 ± 4 37.5 ± 1 Eluate (2.5 ml) 27.5 ± 8 <1.9 11.7 ± 6 Recovery (%) 81 ± 9   85 ± 10  84 ± 9 Uncertainty in the activity (2 σ).

EXAMPLE 3 Testing of the Range of the Method

Analytical methods developed by a pharmaceutical company must bevalidated. The method should be validated in the range from reportinglimit to at least 120% of the specification limit. In the previousexperiments, the amount of ²²⁵Ac has been added related to thespecification limit. As the specification limit is 0.004% relative to²²³Ra, approximately 600 Bq of ²²⁵Ac has been added to 15 MBq ²²³Ra. Thepurpose of Example 3 was to add lower and higher activities of ²²⁵Ac.The reason was to cover the range of amounts of ²²⁷Ac which would beused during validation. Three experiments were conducted with variousamounts of ²²⁵Ac.

Results

Percent recovery of ²²⁵Ac obtained after elution of 10 ml 0.05 mol/LHNO₃ were 91, 74 and 92%. The results are presented in Table 7. Thisshows that the recovery is acceptable (>70%) from 46 to 172% of thespecification limit.

TABLE 7 Measured ²²⁵Ac activity (Bq). Three experiments with an ²²⁵Acactivity ranging from 46-172% of the specification limit of ²²⁷Ac.Experiment I Experiment II Experiment III ²²⁵Ac Spike 1033.3 ± 64 644.1± 32 278.3 ± 15 Eluate (5 ml)  872.8 ± 58 459.9 ± 38 241.0 ± 26 Eluate(5 ml)  70.5 ± 14 15.7 ± 6 15.7 ± 6 Recovery (%)   91 ± 8  74 ± 7   92 ±12 The uncertainties are given as 2 σ.

EXAMPLE 4 Determination of Counting Conditions

Separation of ²²⁷Ac, ²²⁷Th and ²²³Ra was performed by UTEVA and DGAcolumns as described in Example 1, with the difference that a sample of534±21 Bq (2σ) ²²⁷Ac was added to the column rather than the ²²⁵Ac spikeand supplemental ²²⁷Th. Prior to separation, the sample was counted witha HPGe detector and quantified via its daughter ²²⁷Th as ²²⁷Ac was inequilibrium with its daughters for this sample.

Results

²²⁷Ac was eluted from the DGA column with 0.05 mol/L HNO₃ and counted inthe calibrated position 5 cm from the detector surface for 10000 s afterapproximately 1 and 2 days. Results are given in Table 8.

TABLE 8 Results from γ-ray spectrometry ingrowth of ²²⁷Th from ²²⁷Ac.Activity Deviation from Calculated Measured activity Calculated Days(Bq) (Bq) (%) 1.08 21.0 ± 0.8 21.3 ± 1.2 −1.4 ± 0.8   2.08 39.7 ± 1.539.2 ± 2.8 1.3 ± 1.5 The uncertainties are given as 2 σ.

As seen from Tabl, satisfactory counting uncertainties (<7%, 2σ) wereachievable for a sample counted for 10000 s in position 5 cm from thedetector. There was no difference between calculated and measuredactivity.

EXAMPLE 5 Validation of the Method

Analytical methods developed by a pharmaceutical company must bevalidated. Analytical method validation is the process to confirm thatthe analytical procedure employed for a specific test is suitable forits intended use, i.e. to ensure reliability, consistency and accuracyof the analytical data. In order to demonstrate the applicability of themethods of the invention to commercial applications it was validatedaccording to ICH Harmonized Guideline. Prior to a formal methodvalidation it is mandatory to set up a protocol with test parameters tobe evaluated and appropriate acceptance criteria.

The method was validated in terms of selectivity, accuracy, precision(repeatability/intermediate precision), linearity, range, limit ofdetection (LOD) and limit of quantification (LOQ). Robustness of themethod was performed in Example 2 in terms of different lots of resinsand was hence was not repeated in this Example.

The ICH guideline says nothing about acceptance criteria for thedifferent parameters. However, accuracy in terms of recovery between80-120% and a precision of ±20% is normally regarded as acceptable. Thisis applicable for impurities >0.1% of the active ingredient. As thespecification for the impurity ²²⁷Ac in ²²³Ra-chloride drug substance isset as low as 0.004% relative to ²²³Ra, a broader acceptance wasrequired.

Samples of ²²³Ra-chloride drug substance were spiked with known amountsof ²²⁷Ac and ²²⁷Th. Validation parameters and corresponding acceptancecriteria for the method validation are given in Table 9.

TABLE 9 Validation parameters and acceptance criteria ValidationParameter Acceptance Criteria Selectivity The energies of the γ-rays of²²⁷Th are clearly resolved from the energies of the radionuclidespotentially present in the matrix. Accuracy of ²²⁷Ac as % recovery70-130%   Repeatability 60% of <30% (% RSD) specification (n = 3) 100%of <30% specification (n = 3) 140% of <30% specification (n = 3)Linearity, Correlation coefficient, r >0.95 LOQ (Bq) NA¹ LOD (Bq) NA¹¹NA = Not applicableExperimental Parameters

According to ICH, accuracy and repeatability should be assessed using aminimum of 9 determinations over a minimum of 3 concentration levelscovering the specified range (e.g. 3 concentrations/3 replicates). Therecommended range for validation of an impurity method is from reportinglimit to at least 120% of the specification. Samples from 60-140% of thespecification limit were prepared according to Table 10.

TABLE 10 Overview of samples used in method validation. ²²³Ra is spikedwith ²²⁷Ac and ²²⁷Th. The ²²⁷Ac activity is from 60-140% of thespecification limit. ²²⁷Ac ²²⁷Th ²²³Ra % of Activity Activity Activityspecification (Bq) (kBq) (MBq) Replicates 60 360 75 15 3 80 480 75 15 1100 600 75 15 3 120 720 75 15 1 140 840 75 15 3

The method stipulates a sample size of 15 MBq. According tospecifications, the amount of ²²⁷Ac and the amount of ²²⁷Th should beless than 0.004% and less than 0.5% relative to ²²³Ra activity,respectively. As the decided validation range was from 60 to 140% of thespecification, spikes of ²²⁷Ac from 360 to 840 Bq were prepared. Thecontent of ²²⁷Th was held constant i.e. 75 kBq (0.5% of thespecification). The ²²⁷Ac and ²²⁷Th stock solutions were both made in 4mol/L HNO₃ and the activities were approximately 5 Bq/μL and 0.5 kBq/μL,respectively. To determine the exact activity of ²²⁷Ac spike solutions,counting with a HPGe detector in position 5 cm for 1000 s was performed.The counting time was selected to give a counting uncertainty (1 σ) ofless than approximately 3%, which was regarded as adequate. Counting of²²⁷Th stock solutions was performed in position 20 cm for 300 seconds.

²²³Ra from three ²²³Ra-chloride drug substance batches were pooled.Contamination of ²²⁷Ac in the pooled batch was determined. An aliquot of15 MBq was taken from the pooled sample and analyzed according to themethod described in Example 1. This was done to ascertain if anycorrection needed to be made to the above results on account ofbackground contamination of the samples with ²²⁷Ac. No traces of ²²⁷Acin the pooled ²²³Ra-chloride drug substance were found. Hence, nocorrection was performed.

Results—Selectivity

Selectivity is the ability of the measurement to assess the analytewithout any interference from other components in the matrix. At thetime of analysis, radionuclides present in ²²³Ra-chloride drug substancehave been separated from ²²⁷Ac according to the method presented inExample 1. β-decay of ²²⁷Ac does not produce emissions of γ-rays thatare appropriate for γ-detection. Traces of ²²³Ra and its daughters canremain in the sample after purification and the selectivity of themethod is demonstrated by comparing the energies of the γ-rays of ²²⁷Thwith the energies of ²²³Ra and its γ-emitting daughters, ²¹⁹Rn, ²¹¹Pband ²¹¹Bi, and by showing that they are clearly separated andidentifiable. The γ-ray used for quantification of ²²⁷Th is 236.0 keV,which is the most abundant γ-line of ²²⁷Th (12.9%).

The γ-ray energies characteristic of ²²⁷Th, ²²³Ra and daughters areshown in Table 11. A spectrum acquired 24 hours after separation of²²⁷Ac from ²²³Ra-chloride drug substance is shown in FIG. 5.

TABLE 11 γ-ray energies of ²²³Ra and its daughters and ²²⁷Th ²²³Ra ²¹⁹Rn²¹¹Pb ²¹¹Bi ²²⁷Th 144.2 154.2 210.6 236.0 256.2 269.5 271.2 286.1 300.0304.5 323.9 329.9 338.3 351.1 401.8 404.9 427.1 704.6 832.0 (Data takenfrom Evaluated Nuclear Structure Data File (ENSDF) database)

As seen from FIG. 5, the γ-ray used for quantification of ²²⁷Th isdistinctly and visibly separated from the energies of other nuclides.There are no interferences from the matrix. The method is consideredspecific and the acceptance requirement was fulfilled.

Results—Accuracy

The accuracy of the method was determined by performing recoveryexperiments on ²²³Ra-chloride drug substance spiked with five levels of²²⁷Ac at 60%, 80%, 100%, 120%, and 140% of the specification limit of²²⁷Ac. The solutions were additionally spiked with ²²⁷Th amountscorresponding to the ²²⁷Th specification limit. For the 60%, 100% and140% levels the solutions were prepared in three-fold. For the 80% and120% level the solutions were prepared once.

Solutions were analyzed as described in Example 1. Each solution wasmeasured twice. First measurement was performed 24±1 hour after samplepreparation, the second measurement was performed 48±1 hour after samplepreparation.

Accuracy as the percent recovery was determined using the measured ²²⁷Accontent calculated as described in Equation 4.

$\begin{matrix}{{Recovery} = {\frac{{Measured}\mspace{14mu}{content}}{{Nominal}\mspace{14mu}{content}} \times 100}} & (4)\end{matrix}$Results are presented in Table 12.

TABLE 12 Results for the accuracy (as recovery) of the method. Samplesin the range of 60-140% of the specification limit for ²²⁷Ac relative to²²³Ra. Nominal Calculated Diff between activity ²²⁷Th after ²²⁷Th afteractivity measurement Level ²²⁷Ac 24 hours 48 hours ²²⁷Ac 1 and 2Recovery [%] [Bq]¹ [Bq]¹ [Bq]¹ [Bq]² [Days] [%]² 60 323 ± 22.0 20.4 ±2.7 31.6 ± 3.0 303 ± 4.0 1.0 94.0 ± 6.5 322 ± 22.5 10.9 ± 2.6 25.7 ± 2.9397 ± 3.9 1.0 123.2 ± 8.7  359 ± 23.0 16.9 ± 2.0 26.5 ± 2.3 263 ± 3.01.0 73.3 ± 4.8 80 460 ± 27.6 16.9 ± 2.5 30.5 ± 2.9 369 ± 3.8 1.01 80.3 ±4.9 100 540 ± 30.2 22.1 ± 2.0 42.7 ± 2.8 578 ± 3.4 0.98 107.0 ± 6.0  591± 31.9 24.0 ± 2.1 43.8 ± 2.9 556 ± 3.6 0.97 94.1 ± 5.1 527 ± 29.5 21.1 ±3.2 40.1 ± 4.1 535 ± 5.2 0.97 101.5 ± 5.8  120 715 ± 35.8 26.7 ± 2.353.8 ± 2.9 655 ± 3.7 1.14 91.6 ± 4.6 140 868 ± 39.9 36.0 ± 4.9 62.8 ±5.1 735 ± 7.1 1.00 84.7 ± 4.0 861 ± 41.3 31.0 ± 3.2 56.7 ± 3.6 706 ± 4.81.00 82.0 ± 4.0 803 ± 38.5 36.8 ± 4.6 61.0 ± 4.5 662 ± 6.4 1.00 82.4 ±4.0 Mean recovery [%] (n = 11) 92.2 Relative standard deviation of therecovery [%] (n = 11) 15.5 Confidence interval (95%) of recovery [%]82.2-102.1 ¹Uncertainty in the activity (2 σ). ²Combined and recoveryuncertainty

As seen from Table 12, the single percent recovery and the mean (n=11)is all within the criteria of acceptance (70 to 130%, see Table 9). Themethod is considered sufficiently accurate for the determination of²²⁷Ac content in the range from 60% to 140% of the specification limitwhich corresponds to 0.002%-0.006% of ²²⁷Ac in ²²³Ra-chloride drugsubstance at release. The requirement is thereby fulfilled.

The uncertainties in recovery were in the range from 4-8.7%, comparableto 2σ in the counting statistics, and it was the lowest activity whichgave rise to the largest uncertainties. A contribution to theuncertainties is the uncertainty in the spiked value. This is notrelevant for analyses of “normal” ²²³Ra-chloride drug substance samplesand hence the real uncertainties are lower.

Results—Precision

The repeatability of the method was determined by calculating therelative standard deviation (RSD) for three replicates of ²²⁷Ac in²²³Ra-chloride drug substance at three different levels at 60%(corresponding to 0.002% of ²²⁷Ac), 100% (0.004% of ²²⁷Ac), and 140%(0.006% of ²²⁷Ac) of the specification limit. For each level, thesolutions were prepared in triplicate and analyzed as described inExample 1. Results are presented in Table 13.

As seen from Table 13, the mean relative standard deviations were ≦30%for all three levels. The method is considered sufficiently precise andthe acceptance requirement was fulfilled (see Table 9).

TABLE 13 Results for the precision of the method Recovery Mean RSD Level²²⁷Ac (n = 3) (n = 3) [%] [%] [%] [%] 60 94.0 96.8 25.9 123.2 73.3 100107.0 100.9 6.4 94.1 101.5 140 84.7 83.0 1.7 82.0 82.4Results—Intermediate Precision

Intermediate precision expresses within-laboratory variations in termsof e.g. different days, different analysts and different equipment. Theintermediate precision was in this case determined in terms of differentdays. Separation was performed on four different days and the resultsare given in Table 14.

TABLE 14 Intermediate precision. Measured activity Level Ac-227 [Days][Bq] 1 94.0 1 123.2 2 73.3 3 107.0 3 94.1 3 101.5 4 84.7 4 82.0 4 82.4Mean recovery [%] (n = 9) 93.6 RSD [%] (n = 9) 16.3

As seen from Table 14, the mean relative standard deviation was ≦30% forall four days. Data show that the results from different days arecomparable and that the acceptance requirement are thereby fulfilled.

Results—Linearity

Linearity is the ability to generate a response which is directlyproportional to the concentration of an analyte in a sample. Todemonstrate the linearity of the method, a sample with an activity of359±23 Bq ²²⁷Ac was used. The sample was separated according to theprocedure described in Example 1. The in-growth of ²²⁷Th from ²²⁷Ac wasmeasured 6 times in a period from 1 to 6 days after separation. Thecorresponding theoretical activity was calculated using Equation 1.Results are presented in Table 15 and the plot of signals is displayedin FIG. 6.

TABLE 15 Results for the linearity of the method Time from TheoreticalMeasured activity separation ²²⁷Th ²²⁷Th [Days] [Bq] [Bq] 1.0 13.2 ± 0.816.9 ± 2.0 2.0 25.8 ± 1.6 26.5 ± 2.3 2.4 30.2 ± 2.0 30.5 ± 2.4 3.1 39.1± 2.5 43.7 ± 1.5 4.3 53.0 ± 3.4 48.0 ± 3.0 5.9 70.6 ± 4.5 67.0 ± 3.5Regression line Slope 0.8629 Intercept [Bq] 5.4606 Correlationcoefficient (r) 0.9797The linearity curve was measured with ²²⁷Th activities ranging from17-67 Bq. This range covers the ²²⁷Th activities which are measured fromdecay of ²²⁷Ac in specification levels of 78-156% (100% gives anactivity of 21.9 Bq after 24 hours and 42.9 Bq after 48 hours, see Table1). The measured ²²⁷Th activity is plotted as a function of thetheoretical ²²⁷Th activity. The correlation coefficient was determinedto be r=0.98 and is well above the criteria of acceptance (≧0.95). Themethod gives a linear response and the requirement is fulfilled (seeTable 9).Results—Range

The method is validated in the specific range of ²²⁷Ac content from0.002% to 0.006% relative to ²²³Ra with respect to activity (Bq).Linearity, accuracy, and precision of the method were demonstrated overa range of ²²⁷Ac amounts as listed in Table 16.

TABLE 16 Tested range for linearity, accuracy, and precision of themethod Validation ²²⁷Ac characteristics [%] Linearity 0.003% to 0.006%Accuracy 0.002% to 0.006% Precision 0.002%, 0.004%, and 0.006%Results—Limit of Quantification and Limit of Detection

A part of a formal validation of the method is to determine limit ofdetection (LOD) and limit of quantification (LOQ). Limit of blank (LOB)is the highest apparent analyte concentration expected to be found whenreplicates of a blank sample containing no analyte are tested. LOD isthe lowest analyte concentration likely to be reliably distinguishedfrom the LOB and at which detection is feasible. LOQ is the lowestamount one can quantify with sufficiently good (and preselected)accuracy and precision. The γ-peak, 236 keV, which is the most abundant²²⁷Th peak, is used for quantification of the ingrowth of ²²⁷Th from²²⁷Ac. The LOD and LOQ were determined as described using the equations:

$\begin{matrix}{{LOD} = {2.71 + {3.29\left( {B\left( {1 + \frac{n}{2m}} \right)}^{\frac{1}{2}} \right)}}} & (5) \\{{LOQ} = {50\left( {1 + \left( {1 + \frac{nB}{25m}} \right)^{\frac{1}{2}}} \right)}} & (6)\end{matrix}$Where:

-   n=Number of channels in the peak region-   m=Number of channels used for the background estimation-   B=Background correction

Calculated LOD and LOQ from equations 5 and 6 are given in counts andthe corresponding activity (Bq) was calculated using the followingequation:

$\begin{matrix}{A_{E} = \frac{N_{E}}{ɛ_{E} \cdot t \cdot \gamma}} & (7)\end{matrix}$

-   A_(E): The activity in Bq of a nuclide based on a γ-peak with energy    E-   N_(E): the net peak area for a γ-peak at energy E (counts)-   ε_(E): the detector efficiency at energy E-   γ: emission probability-   t: counting time

The LOD was calculated to be 1.8 Bq. This corresponds to 8% of thespecification limit as the ingrowth after 24 hours from 100% ofspecification (600 Bq) corresponds to 22 Bq. The method is suitable todetect an ²²⁷Ac content of 0.0003% (LOD). The LOQ was calculated to 7 Bqof ²²⁷Th this corresponds to 32% of the specification limit. The methodis suitable to quantify an ²²⁷Ac content of 0.0013% (LOQ).

A summary of the validation results is presented in Table 17. Accuracyand precision were assessed on drug substance sample solutions spikedwith ²²⁷Ac activity ranging from 60 to 140% of the specification limit.100% of the specification limits corresponds to 0.004% ²²⁷Ac relative to²²³Ra.

TABLE 17 Summary of validation results Samples (% of AcceptanceValidation Parameters specification) Criteria Results Accuracy as %recovery Spiked samples 70-130%   92.2% (average, n = 11) from 60-140%Correlation coefficient, r Samples from >0.95   0.98 78-156%Repeatability 60% of Spiked samples <30% 25.9% (% RSD) specification(60%) (n = 3) 100% of Spiked samples <30%  6.4% specification (100%) (n= 3) 140% of Spiked samples <30%  1.7% specification (140%) (n = 3) LOQ(Bq) Spiked samples NA 7 LOD (Bq) Spiked samples NA 2

As seen from Table 17, LOD is 2 Bq and LOQ is 7 Bq. This corresponds toapproximately 8% and 32% of the specification limit, respectively.Investigation of the specificity shows that the γ-ray energy forquantification of ²²⁷Th from ²²⁷Ac is clearly resolved from interferingγ-ray energies. There are no interferences from the matrix.

All validation parameters met the pre-specified acceptance criteria. Themethod is considered suitable for its intended use.

The invention claimed is:
 1. A method for the quantification of ²²⁷Ac ina ²²³Ra composition, said method comprising: (i) passing said ²²³Racomposition through a first solid phase extraction column A, whereinsaid column A comprises a thorium specific resin; (ii) passing theeluate of column A through a second solid phase extraction column B,wherein said column B comprises an actinium specific resin; and (iii)recovering the ²²⁷Ac absorbed onto the resin in column B and determiningthe amount thereof.
 2. A method as claimed in claim 1, wherein thethorium specific resin comprises a phosphonate extractant.
 3. A methodas claimed in claim 2, wherein the phosphonate extractant is an alkylphosphonate extractant.
 4. A method as claimed in claim 1, wherein thethorium specific resin comprises a dialkyl alkyl phosphonate extractantof Formula I:

wherein each of R₁-R₃ is independently a C₃-C₈ straight or branchedchain alkyl group.
 5. A method as claimed in claim 4, wherein thedialkyl alkyl phosphonate extractant is a dipentyl pentylphosphonateextractant.
 6. A method as claimed in claim 1, wherein the actiniumspecific resin comprises a diglycolamide extractant.
 7. A method asclaimed in claim 1, wherein the actinium specific resin comprises atetra-alkyl diglycolamide extractant of Formula II:

wherein R₁-R₄ are independently C₃-C₁₂ straight or branched chain alkylgroups.
 8. A method as claimed in claim 7, wherein the tetra-alkyldiglycolamide extractant is a N,N,N′,N′-tetra-n-octyldiglycolamide (DGA)extractant.
 9. A method as claimed in claim 1, wherein column A andcolumn B are arranged in series.
 10. A method as claimed in claim 1,wherein the eluent used in both columns A and B comprises aqueous nitricacid.
 11. A method as claimed in claim 1, wherein recovery of the ²²⁷Acin step (iii) is achieved by washing column B with an aqueous acid. 12.A method as claimed in claim 11, wherein the washing volume of theaqueous acid is 16 to 400 times the volume of column B.
 13. A method asclaimed in claim 12, wherein the washing volume of the aqueous acid is40 to 200 times the volume of column B.
 14. A method as claimed in claim1, wherein the determination in step (iii) is achieved by γ-spectrometryvia in-growth and detection of the daughter ²²⁷Th.
 15. A method asclaimed in claim 1, said method comprising: (i) Adding a volume of a²²³Ra composition corresponding to a known activity of ²²³Ra to an equalvolume of nitric acid; (ii) Transferring the sample from step (i) to theinput of a first solid phase extraction column A comprising a thoriumspecific resin arranged in series with a second solid phase extractioncolumn B comprising an actinium specific resin; (iii) Passing saidsample through both columns A and B; (iv) Washing both columns withnitric acid in an amount of 20-100 times the combined volumes of the twocolumns; (v) Disconnecting column A from column B; (vi) Washing column Bwith nitric acid in an amount of 40-200 times its volume; (vii) Washingcolumn B with nitric acid in an amount of 40-200 times its volume at aconcentration less than that used in step (vi); and (viii) Determiningthe amount of ²²⁷Ac present in the eluate from column B obtained in step(vii).
 16. A method as claimed in claim 15, wherein the determination instep (viii) is achieved by γ-spectrometry via in-growth and detection ofthe daughter ²²⁷Th.
 17. A method as claimed in claim 15, wherein thenitric acid in steps (iv) and (vi) is 4 mol/L nitric acid.
 18. A methodas claimed in claim 15, wherein the nitric acid in step (vii) is 0.05mol/L nitric acid.